Moisture indicator for photoflash lamp

ABSTRACT

A photoflash lamp containing a moisture indicator in the form of a coating of dried paste comprising a mixture of cobaltous cabalticyanide, a colloidal magnesium silicate, and between about 0.5 percent and about 2 percent by weight of partially hydrolyzed polyvinyl alcohol.

United States Patent Painter et al.

[451 Feb. 11, 1975 3,195.326 7/1965 Shaffer 431/13 $536,471 6/1971 McDonough el al. 431/13 3,609,331 9/1971 Fink et a1 431/13 3,770,362 1 1/1973 Shaffer 3.816.054 6/1974 Baldrige et a1 431/95 Primary Examiner-Carroll B. Dority, Jr. Attorney, Agent, or Firm-Edward .1. Coleman [57] ABSTRACT A photoflash lamp containing a moisture indicator in the form of a coating of dried paste comprising a mixture of cobaltous cabalticyanide, a colloidal magnesium silicate, and between about 0.5 percent and about 2 percent by weight of partially hydrolyzed polyvinyl alcohol.

16 Claims, 3 Drawing Figures MOISTURE INDICATOR FOR PHOTOFLASH LAMP BACKGROUND OF THE INVENTION This invention relates in general to the manufacture of photofiash lamps and more particularly to flashlamps having means for determining the presence of air contamination within the sealed lamp envelope.

Photoflash lamps generate their actinic light output by the burning of an energetic fuel, such as finely shredded zirconium or hafnium metal foil, in a combustion supporting atmosphere. such as oxygen. The containing vessel, or envelope, for such combustion must be transparent and must be hermetically sealed so that a chosen atmospheric composition at a chosen stoichiometric balance will be present. Such lamp envelopes are most commonly made from glass. A crack or other defect in the envelope destroys its hermcticity and the resulting air contamination adversely affects the light output and timing characteristics of the fiash' lamp. Accordingly, it is common practice in the manufacture of photoflash lamps to apply a spot of moisture indicating material to the inner surface of the envelope to indicate, by a color change, whether or not the hermetic closure is intact. The material generally used for this purpose is cobaltous cobalticyanide, which in the anhydrous state is blue and which becomes pink when hydrated. The water vapor normally present in air effects a spot color change from blue to pink when the hermetic seal of the lamp envelope is broken.

As cobaltous cobalticyanide is a fine, water insoluble powder, it is mixed with a binder so that it will adhere to an interior surface of the sealed flashlamp. For example, U.S. Pat. No. 3.l95,326, assigned to the present assignee. describes a moisture indicator material containing an inorganic binder of colloidal magnesium silicate, while US. Pat. No. 3,770,362, also assigned to the present assignee, describes an indicator material containing an organic binder of partially hydrolized polyvinyl alcohol. A typical method of applying the indicator material during the lamp manufacturing process is by use of a wire dip stick which is initially dipped into a fluid mixture of the cobaltous eobalticyanide and binder and then inserted into the glass envelope of the lamp to deposit a spot of the indicating paste on the inside surface thereof.

In producing subminiature lamps having envelope volumes of less than, say, 0.5 cc., however, the dip stock spotting method of application can become quite difficult. Accordingly, a copending application Ser. No. 406,389, filed Oct. 15, 1973 and assigned to the present assignee, describes an improved method of application for subminiature size. electrically ignitable lamps whereby an inorganic indicator material is coated on the ignition mount structure instead of the envelope wall. The several advantages of this latter approach may best be explained as follows.

in some of the tubular electrically ignitable photoflash lamps presently manufactured, the ignition means comprises a pair of lead-in wires sealed through one end of the tubular glass envelope and supported in a spaced side-by-side relation by a glass bead fused about the wires. Certain other lamp types additionally employ a glass insulating sleeve about one of the lead'in wires for preventing post-ignition short circuits. A tungsten filament is mounted across the inner ends of the two leadin wires with the ends of the wires at their junctions with the filament being coated with a primer material, such as a powdered zirconium mixture. When battery current is applied to the external projecting portions of the two lead-in wires, the filament glows to incandescence, causing the primer material to ignite, which in turn ignites the finely shredded metallic combustible material in the lamp to produce a predetermined quantity of light output.

The primer on the ends of the lead-in wires is generally applied by a dipping method. A hollow tube containing a liquid mix of the primer may be introduced into the open end of the tubular shaped glass envelope, or the envelope may be lowered over a fixed primer containing tube, until the ignition structure is sufficiently coated with the primer material and then is withdrawn. ln accordance with the aformentioned eopending application, Ser. No. 406,389, an inorganic liquid indicator material is applied by a dip process similar to that of applying the primer and preceding the primer dipping process. The lamp ignition structure is dipped sufficiently deep into the moisture indicating material so that some of the material is deposited on portions of the lead-in wires and the glass insulating sleeve, or on a portion of the glass bead ifno glass insu lating sleeve is used. After sufficient drying of the indicator material, the assembly is then ready for the primer dip application which is applied partly over the indicator material on the ends of the lead-in wires.

By using this indicator material dip method and employing as the moisture indicating material an inorganic mixture substantially comprising cobaltous cobalticyanide and a binder of colloidal magnesium silicate, several additional advantages are provided for significantly enhancing flashlamp operation. Firstly. in this manner of dip application, a coating of the indicator material is also deposited on the two lead-in wires which support the filament above the glass bead. The non-conductive properties of this thin coating of indi' cator material serve to insulate the critical portions of the lead-in wires and, thus, prevent possible electrical short circuit contacts between the lead-in wires and the finely shredded strands of metallic combustible material prior to ignition.

A second advantage is improved vessel containment during lamp flashing. Both the location and composition of the moisture indicating material contribute to this result. By coating the indicator material on the lead-in wires and insulating sleeve or bead instead of spotting the inner wall of the glass envelope, a possible hot spot source, due to oxidizing of the indicator material during ignition, is removed from the glass containing wall. Further, it has been observed that by using a substantially inorganic binder in an otherwise inorganic indicating material, a minimal amount of pressure build-up is produced during the flash cycle as compared to indicating materials employing an organic binder. Minimizing the pressure buildup, to thereby reduce the burden on the containing vessel wall, is particularly desirable in the case of subminiature size lamps, which typically employ relatively high fill pressures.

When the aforementioned inorganic indicator material is thinned with water to the consistency necessary to coat the glass sleeve while still not bridging the lead wires, however, a disadvantage is encountered in that, after lamp fabrication, the moisture indicator is pale blue in color and difficult to detect at present production speeds.

SUMMARY OF THE INVENTION Accordingly, it is an object of this invention to provide a superior moisture indicator for photoflash lamps.

A particular object of the invention is to provide an improved photoflash lamp of the type containing a coating of dried paste for clearly indicating by a change in coloration the presence of air therein.

These and other objects, advantages and features are attained, in accordance with the principles of this invention, by our discovery that by adding a small percentage of organic material, such as partially hydrolyzed polyvinyl alcohol, to a larger quantity of inorganic material, such as a colloidal magnesium silicate, as the binding agent in the moisture indicator paste, we can retain the desirable properties of the inorganic material with respect to minimizing pressure buildup during flashing while significantly improving the color indication. More specifically we have found that by using between about 0.5 percent to about 2 percent by weight of a water soluble organic polymer of the hydrogen bonding type (preferably partially hydrolyzed polyvinyl alcohol) in the dried paste, the dried indicator material exhibits a more intense and easily identified blue color, as compared to the pale blue of a dried mixture of cobaltous cobalticyanide and colloidal magnesium silicate alone. A proportion of polyvinyl alcohol below about 0.5 percent is insufficient to produce the desired color improvement, while a proportion of over 2 percent more significantly detracts from the containment advantages of the inorganic binder material for subminiature flash lamps with high fill pressures.

In addition to color, we have quite unexpectedly observed other characteristics of this inorganic/organic mixture to be changed dramatically from indicators employing as the sole binding agent either colloidal magnesium silicate or partially hydrolyzed polyvinyl alcohol. For example, low viscosity mixtures of colloidal magnesium silicate and cobaltous cobalticyanide salts do not pour well into small diameter containers due to the high surface tension, nor do they sufficiently wet the glass insulating sleeve or bead. On the other hand, low viscosity mixtures of partially hydrolyzed polyvinyl alcohol and cobaltous cobalticyanide salts pour well and wet the glass better, but excessive foaming occurs. Antiform agents can be used but are not entirely effective. When a small percentage of partially hydrolyzed polyvinyl alcohol is added to a mixture of cobaltous cobalticyanide and colloidal magnesium silicate, however, the consistency is markedly changed, viz., it pours well, has body, adheres well to the glass sleeve, and results in a good blue color in finished lamps.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will be more fully described hereinafter in conjunction with the accompanying drawings, in which:

FIG. 1 is an elevational view, partly in section of one type of electrically ignitable photoflash lamp provided with a moisture indicator in accordance with the principles of this invention;

FIG. 2 is an elevational view, partly in section, of another type of electrically ignitable photoflash lamp provided with a moisture indicator in accordance with the invention; and

FIG. 3 is a sectional elevation of a percussive-type photoflash lamp provided with a moisture indicator, according to the invention.

Referring to FIG. I, one embodiment of the invention is illustrated as applied to an electrically ignitable photoflash lamp having a pair of filament-supporting lead-in wires supported by a glass bead. The lamp com prises an hermetically sealed, light transmitting envelope 10 of glass tubing having a press 12 defining one end thereof and an exhaust tip 14 defining the other end thereof. A quantity of filamentary combustible material 16, such as shredded zirconium or hafnium foil, is located within the lamp envelope. The envelope 10 is also provided with a filling of combustionsupporting gas, such as oxygen, at a pressure of several atmospheres.

The ignition means comprises a pair of lead-in wires 18 and 20 extending through and sealed into the press 12. A filament 22 spans the inner ends of the lead-in wires, and beads of primer 24 and 26 are located on the inner ends of the lead-in wires 18 and 20 respectively, at their junctions with the filament. The lead-in wires are supported in a spaced side-by-side relation by a glass bead 28 fused about the wires. As described in the aforementioned copending application, a moisture indicating material 30 is coated on a portion of the glass bead 28 and the portion of the lead-in wires extending from the glass head toward the filament.

FIG. 2 illustrates another embodiment of the invention as applied to a flashlamp identical to that of FIG. l except that it further includes a glass sleeve 32 disposed about a portion of one of the lead-in wires (20) as an insulating shield extending from the glass bead toward the filament for preventing post-ignition short circuits across the lead-in wires. Such a feature is required for the proper operation of certain flash sequencing circuitry for controlling linear arrays of flashlamps. Further, in FIG. 2, a coating of moisture indicating material 34 is disposed on a portion of the glass sleeve 32 and at least portions of the lead-in wires extending toward the filament.

In the manufacture of photoflash lamps of the type described with respect to FIGS. 1 and 2, the lamp envelope 10 is initially a segment of glass tubing, open at both ends. The ignition assembly (or mount structure), which comprises filament 22, lead-in wires 18 and 20 and glass bead 28 (and, in FIG. 2, further includes glass sleeve 32), is located within the glass tubing with the free ends of the lead'in wires projecting from one of the open ends of the tubing. A press seal 12 is then formed to close the ends of the tubing from which the wires project. Next, a portion of the ignition assembly within the glass tubing is dipped into a liquid preparation of moisture indicating material contained in a tube which fits through the open end of the glass tubing. That is, a hollow tube containing a liquid mix of the indicator material may be introduced into the remaining open end of the glass tubing, or the glass tubing may be lowered over a fixed tube containing the indicator liquid. The ignition assembly is dipped sufficiently deep into the liquid indicator material so that some of the material is deposited on the glass bead 28, or, in the lamp of FIG. 2, it may be dipped fully or partially onto the glass sleeve 32. After sufficient drying of the indicator material coatings 30 or 34, the end portions of the leadin wires within the glass tubing are dipped into a liquid preparation of primer, which is applied partly over the indicator material on the lead-in wires. Thereafter, the primer beads 24 and 26 are dried and a charge of combustible material, such as shredded zirconium or hafnium foil, is introduced into the remaining open end of the tubing. The glass is then constricted at the open end, and the envelope is exhausted, filled with oxygen gas at several atmospheres pressure. and tipped off at 14 to define an hermetically sealed envelope.

Thereafter, the coatings of dried paste 30 or 34, which normally have a blue appearance, function to indicate, by a change in coloration to pink, the presence of an air leak in the envelope. The coatings on bead 28 and sleeve 32 are particularly suitable as indicators as they are more visible. In addition, the nonconductive indicator material 30 or 34 insulates critical portions of the lead-in wires l8 and 20 sufficiently to prevent preignition short circuits through the metallic shreds 16. There is a little liklihood of a short below the glass bead as the mass of shreds 16 are generally disposed above bead 28.

In accordance with the present invention, the dried paste of the moisture indicator material comprises a mixture of: cobaltous cobalticyanide; a colloidal magnesium silicate, preferably magnesium montmorillonite, in an amount between about percent to about 30 percent by weight; and, between about 0.5 percent to about 2 percent by weight of a water soluble organic polymer of the hydrogen bonding type, preferably partially hydrolyzed polyvinyl alcohol. Such a mixture brings out the blue color of the dried paste in a finished lamp, whereby the condition of the indicator is more readily identifiable, while still retaining the containment enhancing features of a completely inorganic mixture, viz., minimizing pressure buildup within the lamp subsequent to ignition. In addition, as discussed hereinbefore in the summary, low viscosity mixtures of this indicator material have improved pouring and wetting characteristics, and exhibit a desired reduction in foaming.

While we prefer to use magnesium montmorillonite as the primary component of the binding agent, other colloidal magnesium silicates, such as, for example, lithium-magnesium silicate and aluminum-magnesium silicate, may also be employed satisfactorily.

Polyvinyl alcohol is prepared by hydrolytic removal of acetyl groups from polyvinyl acetate. Two types of polyvinyl alcohol are commercially availablecompletely and partially hydrolyzed grades. The completely hydrolyzed grades have 97 percent or more of the acetyl groups removed, whereas the partially hydrolyzed grades retain from 10 percent to percent of the acetyl groups. The completely hydrolyzed grades of polyvinyl alcohol provide good spot adhesion and abrasion resistance, but when employed as the sole binding agent, the moisture sensitivity of the completely dried out spot is poorer. When employed as a small percentage additive to a primarily inorganic binding agent, however, it provides the desired enhancement of the color and consistency of the indicator material without effecting moisture sensitivity to any noticeable degree. Accordingly, completely hydrolyzed polyvinyl alcohol may be used in the present application in lieu of the partially hydrolyzed grade. Likewise, other water soluble organic polymers of the hydrogen bonding type may be used as the organic additive to the binder, even though some may be unsatisfactory as the sole binding agent for a moisture indicator mix. For example, or-

ganic polymers such as animal glue, hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose and polyvinyl pyrolidone may be used.

By way of example, we have found that a highly satisfactory moisture indicator for a lamp such as that of FIG. 2 may be prepared by combining a dry mixture of about 78.38 percent (220 grams) cobaltous cobalticyanide, 20.84 percent (58.5 grams) of magnesium montmorillonite, and 0.78 percent (2.2 grams) of partially hydrolyzed polyvinyl alcohol by weight in an appropriate vehicle, such as distilled water (4l00 milliliters) and 5 percent boric acid solution (60 milliliters). Sufficient water and boric acid is stirred into the mixture to give the desired percent of solids.

At proportions of magnesium montmorillonite less than about l0 percent, the adherence characteristics of the indicator material became unsatisfactory, while proportions higher than about 30 percent result in a liquid mix that is quite thick and thus, difficult to apply in subminiature lamp structures. The organic additive must comprise at least about 0.5 percent of the dried paste in order to provide the desired color enhancement, while proportions greater than about 2 percent more significantly detract from the containment advantages of the inorganic binder with respect to subminiature flashlamps with high fill pressures.

Another embodiment of the invention is illustrated in FIG. 3, which shows a percussive-photoflash lamp comprising a length of glass tubing defining an hermetically sealed lamp envelope 42 constricted at one end to define an exhaust tip 44 and shaped to define a seal 46 about a primer 48 at the other end thereof. The primer 48 comprises a metal tube 50, a wire anvil 52 and a charge of fulminating material 54. A combustible, such as filamentary zirconium 56, and a combustionsupporting gas, such as oxygen, are disposed within the lamp envelope, as described for the electrically ignited lamp. The wire anvil 52 is centered within the tube and is held in place by a circumferential indenture 58 of the tube 50 which laps over the head 60 or other suitable protuberance at the lower extremity of the wire anvil. Additional means, such as lobes 62 on wire anvil 52 for example, may also be used in stabilizing the wire anvil, supporting it substantially coaxial within the primer tube 50 and insuring clearance between the fulminating material 54 and the inside wall of the tube 50. A refractory bead 64 is fused to the wire anvil 52 just above the inner mouth of the primer tube 50 to eliminate bum-through and function as a deflector to deflect and control the ejection of hot particles of fulminating material from the primer.

Although the lamps of FIGS. 1 and 2 are electrically ignited, usually from a battery source, and the lamp of FIG. 3 is percussion-ignitable, the lamps are similar in that in each the ignition means is attached to one end of the lamp envelope and disposed in operative rela tionship with respect to the filamentary combustible material. More specifically, the igniter filament 22 of the flash lamp in either FIG. 1 or FIG. 2 is incandesced electrically by current passing through the metal fila ment support leads l8 and 20, whereupon the incandesced filament ignites the beads of primer 24 and 26 which in turn ignite the combustible 16 disposed within the lamp envelope. Operation of the percussive-type lamp of FIG. 3 is initiated by an impact onto tube 50 to cause deflagration of the fulminating material 54 up through the tube to ignite the combustible 56 disposed within the lamp envelope.

in the manufacture of the photoflash lamp of FIG. 3, the lamp envelope 42 is initially a segment of glass tubing, open at both ends. in the normal sequence of operations, the mount structure, which comprises the primer 48, is positioned in one of the open ends of the glass tubing and the seal 46 is formed, thereby closing one of the open ends of the tubing. In this instance, a suitable applicator is then inserted into the remaining open end of the tubing to provide the inner wall thereof with a moisture indicator spot, denoted as 66 in FIG. 3, the paste spot 66 being the same composition as the indicator coatings 30 and 34 in the lamps of FIGS. 1 and 2 and providing similar advantages. Thereafter, the paste spot 66 is dried, and a charge of combustible material, in this instance shredded zirconium foil, is introduced into the remaining open end of the tubing. The glass is then constricted at the open end, and the envelope is exhausted, filled with oxygen gas at several atmospheres pressures, and tipped off at 44 to define an hermetically sealed envelope.

Although the invention has been described with respect to specific embodiments, it will be appreciated that modifications and changes may be made by those skilled in the art without departing from the true spirit and scope of the invention.

What we claim is:

l. A photoflash lamp comprising:

an hermetically sealed, light-transmitting envelope;

a quantity of filamentary combustible material located within said envelope;

a combustion-supporting gas in said envelope;

ignition means attached to said envelope and disposed in operative relationship to said combustible material; and

a moisture indicator comprising a quantity of dried paste disposed in said envelope to indicate by a change in coloration the presence of air therein, said dried paste comprising a mixture of cobaltous cobalticyanide, a colloidal magnesium silicate, and between about 0.5 percent to about 2 percent by weight of a water soluble organic polymer of the hydrogen bonding type.

2. The lamp of claim I wherein said organic polymer is polyvinyl alcohol.

3. The lamp of claim 2 wherein said polyvinyl alcohol is a partially hydrolyzed grade.

4. The lamp of claim 1 wherein said colloidal magnesium silicate is magnesium montmorillonite.

5. The lamp of claim 1 wherein said quantity of dried paste contains between about percent to about 30 percent by weight of said colloidal magnesium silicate.

6. The lamp of claim 5 wherein said mixture comprises about 78.38 percent of cobaltous cobalticyanide, about 20.84 percent of magnesium montmorillonite, and about 0.78 percent of partially hydrolyzed polyvinyl alcohol by weight.

7. The lamp of claim 1 wherein said quantity of dried paste is disposed as a spot on the inside surface of said envelope.

8. The lamp of claim 1 wherein said ignition means includes a pair of lead-in wires sealed through one end of said envelope and extending inside said envelope, and said quantity of dried paste is coated on portions of said lead-in wires inside said envelope.

9. The lamp of claim 8 wherein said ignition means further includes a filament disposed within said envelope and attached to said lead-in wires, beads of primer located on the inner ends of said wires at the junctions between the lead-in wires and the filament, and a glass bead fused about said lead-in wires for supporting said wires in a spaced side-by-side relation, and wherein said quantity of dried paste is coated on a portion of said glass bead and the portions of said lead-in wires extending from said glass bead toward said filament.

10. The lamp of claim 9 wherein said quantity of dried paste contains between about 10 percent ot about 30 percent by weight of magnesium montmorillonite as said colloidal magnesium silicate.

11. The lamp of claim 10 wherein said organic polymer is polyvinyl alcohol.

12. The lamp of claim 8 wherein said ignition means further includes a filament disposed within said envelope and attached to said lead-in wires, beads of primer located on the inner ends of said wires at the junctions between the lead-in wires and the filament, and a glass sleeve disposed about a portion of at least one of said lead-in wires as an insulating shield for preventing postignition short circuits across said pair of leadin wires, and wherein said quantity of dried paste is coated on a portion of said glass sleeve and at least a portion of said lead-in wire extending toward said filament.

13. The lamp of claim 12 wherein said ignition means further includes a glass bead fused about said lead-in wires for supporting said wires in a spaced side-by-side relation, and said glass sleeve is located to extend from said glass bead toward said filaments.

14. The lamp of claim 13 wherein said quantity of dried paste contains between about l0 percent to about 30 percent by weight of magnesium montmorillonite as said colloidal magnesium silicate.

15. The lamp of claim 14 wherein said organic polymer is polyvinyl alcohol.

16. The lamp ofclaim 15 wherein said polyvinyl alcohol is a partially hydrolyzed grade 

1. A photoflash lamp comprising: an hermetically sealed, light-transmitting envelope; a quantity of filamentary combustible material located within said envelope; a combustion-supporting gas in said envelope; ignition means attached to said envelope and disposed in operative relationship to said combustible material; and a moisture indicator comprising a quantity of dried paste disposed in said envelope to indicate by a change in coloration the presence of air therein, said dried paste comprising a mixture of cobaltous cobalticyanide, a colloidal magnesium silicate, and between about 0.5 percent to about 2 percent by weight of a water soluble organic polymer of the hydrogen bOnding type.
 2. The lamp of claim 1 wherein said organic polymer is polyvinyl alcohol.
 3. The lamp of claim 2 wherein said polyvinyl alcohol is a partially hydrolyzed grade.
 4. The lamp of claim 1 wherein said colloidal magnesium silicate is magnesium montmorillonite.
 5. The lamp of claim 1 wherein said quantity of dried paste contains between about 10 percent to about 30 percent by weight of said colloidal magnesium silicate.
 6. The lamp of claim 5 wherein said mixture comprises about 78.38 percent of cobaltous cobalticyanide, about 20.84 percent of magnesium montmorillonite, and about 0.78 percent of partially hydrolyzed polyvinyl alcohol by weight.
 7. The lamp of claim 1 wherein said quantity of dried paste is disposed as a spot on the inside surface of said envelope.
 8. The lamp of claim 1 wherein said ignition means includes a pair of lead-in wires sealed through one end of said envelope and extending inside said envelope, and said quantity of dried paste is coated on portions of said lead-in wires inside said envelope.
 9. The lamp of claim 8 wherein said ignition means further includes a filament disposed within said envelope and attached to said lead-in wires, beads of primer located on the inner ends of said wires at the junctions between the lead-in wires and the filament, and a glass bead fused about said lead-in wires for supporting said wires in a spaced side-by-side relation, and wherein said quantity of dried paste is coated on a portion of said glass bead and the portions of said lead-in wires extending from said glass bead toward said filament.
 10. The lamp of claim 9 wherein said quantity of dried paste contains between about 10 percent ot about 30 percent by weight of magnesium montmorillonite as said colloidal magnesium silicate.
 11. The lamp of claim 10 wherein said organic polymer is polyvinyl alcohol.
 12. The lamp of claim 8 wherein said ignition means further includes a filament disposed within said envelope and attached to said lead-in wires, beads of primer located on the inner ends of said wires at the junctions between the lead-in wires and the filament, and a glass sleeve disposed about a portion of at least one of said lead-in wires as an insulating shield for preventing post-ignition short circuits across said pair of lead-in wires, and wherein said quantity of dried paste is coated on a portion of said glass sleeve and at least a portion of said lead-in wire extending toward said filament.
 13. The lamp of claim 12 wherein said ignition means further includes a glass bead fused about said lead-in wires for supporting said wires in a spaced side-by-side relation, and said glass sleeve is located to extend from said glass bead toward said filaments.
 14. The lamp of claim 13 wherein said quantity of dried paste contains between about 10 percent to about 30 percent by weight of magnesium montmorillonite as said colloidal magnesium silicate.
 15. The lamp of claim 14 wherein said organic polymer is polyvinyl alcohol.
 16. The lamp of claim 15 wherein said polyvinyl alcohol is a partially hydrolyzed grade. 